There are a great many studies now being performed to design the next generation ground-based telescope such as CELT (reference 1), 20/20 (reference 2), Euro-50 (reference 3), and OWL (reference 4). All of the projects other than 20/20 have a preliminary design with segmented optics similar to the Keck telescope (reference 5) and Hobby-Eberly telescope (reference 6) for the primary mirror design. Regardless of the design, the bottom-line requirement for any new 30-100 meter class telescope is a low-as-possible cost and time to build, so that the project becomes feasible. The conservative approach (of CELT, Euro-50, OWL, etc.) has been to assume the mirrors will be stress figured and polished as was done for the Keck Observatory primary mirror.
Two general approaches to making segmented telescopes have been explored, but only one put into practice for astronomical telescopes. The one that has been put into practice is the use of relatively massive large segments for Keck (1.8 meters in diameter corner-to-corner, 75 mm thick; segment mass 400 kg, aerial density of 190 kg/m2). Each of the 36 Keck segments has its own set of actuators and edge sensors that are adjusted to take out changes in the total mirror figure due to gravity and changes in the orientation of the telescope. It has been found that the system is stable enough that the phasing takes about 1 hour from start to finish and lasts for several weeks. A modified design that uses only spherical segments such as Hobby-Eberly, uses about 1 meter segments and corrects “slow” (days to weeks) changes of the combined segmented primary mirror figure.
The “Keck” approach is “brute force” and requires adaptive optics downstream to achieve the ultimate power of the telescope. A modified version of this approach is to use even larger segments (e.g. 20/20) but effectively this is the same general concept: apply “slow” corrections (for changes that are mechanically/gravitationally induced) to the primary and apply all “fast changes” (for atmospheric turbulence corrections) downstream. At the other extreme, which has not yet been put into use for large (approximately >2 m diameter) astronomical telescopes, is PAMELA (reference 10). PAMELA uses 7 cm segments (8 mm thick and mass of 40-45 gm for an areal density of about 10 kg/m2) and is rapid enough (150 Hz) to correct for variations that affect the performance in the visible (about 600 nm). On one hand, the PAMELA project demonstrated that the basic concept works. On the other hand, the work on PAMELA demonstrated that there are many details that need to be worked out, such as the damping of the pieces and deformation of the figure when actuators are attached. The PAMELA test bed was only a 0.5 meter mirror with 7 cm segments. At least two key issues have to be resolved: the mounting/alignment and keeping the entire segment stiff enough so that 100 KHz AO closed loop control will have a negligible effect on the shape of the segment figure.
There are several areas in which the basic Keck-like approach could be significantly improved to reduce cost. Some examples of areas of improvement are: lighter weight segments to reduce both actuator costs and telescope mount costs; a smaller f number to reduce the dome and other related costs; a different segment fabrication from that used for the Keck Observatory telescope to reduce costs and “time-to-complete.”
Since the next generation of optical/IR telescopes will require large numbers of co-phased mirror segments, some form of replication technology is desirable to reduce costs. Electroforming has the advantage that it is a commercially developed technology for replication, and the technology has been widely used for making X-ray mirrors (e.g. XMM-Newton) The use of replication in optics has a long, rich history which has been built upon to this day (see below) by both the X-ray and the optical (primarily) space communities.
Electroforming has been able to achieve λ/2-λ/4 at 632 nm, but little has been published in the area of making segments for optical (“normal incidence” optics) mirrors beyond what is in certain web pages (references 12, 13, 14) and J. Denton et al. “Replication of Optical Surfaces: Capabilities and Cautions, in Proceedings of Precision Fabrication and Replication, ASPE, 1999 (reference 11). Much more extensive work has been published for grazing incidence optics, e.g. see references 15-17 and references therein. Replication by composites work has also been carried out extensively (cf. references 18, 19 and references therein), and again, about λ/2-λ/4 is the best that has been achieved.
The work on “normal” incidence mirrors can be summarized as follows. Replication offers a significant advantage over classic grinding and polishing, in principle. The advantage is especially true if spherical mirrors are used, but this is not necessary. The figure quality that can be achieved for aspheres has been quoted as about λ/2-λ/4 at 632 nm, which is marginally acceptable at 2 μm or longer. The issue of support and mounting for a specific system has not been fully addressed. Workers in the art have tended to focus on one technology such as: composites, electroformed metals, or injection molded Pyrex.
Composite mirrors having two layers have been described by O. Citterio et al. in “Development of Soft and Hard X-ray optics for astronomy”, Proceedings of SPIE. Vol. 4138, page 43 (2000) and by O. Citterio et al. in “Development of Soft and Hard X-ray optics for astronomy”, Proceedings of SPIE, Vol. 4496, page 23 (2002), and by R. Hudec et al, “Light-weight X-ray optics for future space missions”, Proceedings of SPIE, Vol. 4851, page 656 (2003). The composite mirrors are comprised of two layers; i.e. a reflective layer and a ceramic shell. The reflective layer and the ceramic shell are produced separately and then attached to each other by epoxy. There are unbalanced residual stresses in these mirrors due to the fact that the mirrors consist of only two layers. As a result, stiffening rings have to be glued on the ceramic shell external surface to correct out-of-roundness errors that are caused by the presence of residual stresses in the materials.